This invention relates generally to the field of high pressure technology and more particularly to apparatus and methods for the rapid attainment of high hydrostatic pressures for concurrent use in processing workpieces. More specifically, this invention relates to apparatus and methods for processing various workpieces of a metallic and nonmetallic nature at high hydrostatic pressures with improved cost efficiency for the generation and use of such pressures.
Considerable effort is presently expended in the design, manufacture, engineering and utilization of advanced materials. As the technological demands of modern society increase, so too does the need increase for superior materials meeting these requirements. A vital component in the development of new materials is the development and economic use of processes which improve the properties of otherwise inadequate materials. Such processes, and the associated equipment for carrying them out, are the subject of the present invention.
One of the major methods for processing materials to achieve improved performance involves the application of high pressures, frequently (but not always) in conjunction with high temperatures. For example, increasing commercial applications are appearing for the products of "powder metallurgy". This technology involves the processing of one or more powdered components including specific metals in specific amounts to form solid components of superior performance. Typically, such powdered materials are compressed under high pressure (often in conjunction with high temperature), to produce a solid metal material with the desired properties.
Other applications of high pressure involve "hot isostatic pressing" ("HIP") in which heat and pressure are applied to a variety of materials (metals and nonmetals) to achieve a variety of ends. The field of HIP is so diverse that here we can only describe a few of the salient applications. For example, many metals fabricated by conventional metal-forming technology have voids or spaces within the structure of the fabricated material. That is, the piece as fabricated does not possess the full density of the material itself. HIP is commonly used in the "densification" of such materials to remove potentially harmful voids. Densification need not be confined to the processing of metals, as many plastic, ceramic and composite materials also form voids in fabrication. The densification by HIP (or sometimes by pressure alone without the use of heat) is an emerging technology for improving the properties of such nonmetals also.
Another common application of HIP technology is in the formation of bonds between materials which are difficult or impossible to join by any other technique. HIP has successfully bonded surface claddings of costly, high performance materials onto relatively inexpensive substrates, achieving thereby considerable increase in performance at a relatively modest addition in cost. More applications of HIP in surface cladding, alloying and joining are being reported all the time.
Because of the commercial importance of high pressure processing of various materials described above, considerable research and engineering has gone into the development of economical processes and equipment for the attainment of the high hydrostatic pressures required in such processes. There are a number of problems to be considered. For example, many of the workpieces requiring the application of high pressure are large. Components of jet aircraft engines are a typical example of reasonably large workpieces (several feet) requiring high pressure processing. Such jet engine components are currently receiving HIP processing, since the advantages of HIP processing for such costly pieces outweigh the difficulties of making HIP equipment for processing large-volume workpieces.
In addition to the example of jet aircraft engines, there are many potential areas for high pressure processing of bulk steels and other materials of major potential commercial application. Such processes are well studied in the laboratory and known to produce very favorable results. The barrier to commercialization has been the lack of a technology for applying high pressures to such large samples at a commercially acceptable price.
The technical and patent literature abound in references to high pressure equipment and processes which have not, and cannot, be applied to the large specimens of industry at reasonable costs. It is a major goal of the present invention to describe a technology which does lend itself to the processing of commercial-size workpieces at acceptable costs.
Another problem associated with the commercial high pressure processing of large samples is the cost of the process. Industry must deal with a fundamental dilemma in processing large workpieces. To apply high pressures to a large volume, the cost of the pressure-producing equipment is typically quite large. To lower the costs of capital equipment, high pressure equipment is typically purchased which attains the high pressures only slowly. That is, the equipment is the smallest (and cheapest) which does the job but, of necessity, it does the job slowly. Thus, the workpieces to be processed must first await the attainment of high pressure, receive the required processing time at that pressure, and then exit the machinery. The slow attainment of high pressure markedly slows down many processes, reducing the number of workpieces that can be processed each day, and inherently driving up the effective cost of processing each piece. The conventional solution to such problems of "cycle time" is to use pressure equipment having excess capabilities, well above that required for the job at hand. Such over-capacity can achieve the working pressure much more quickly and, therefore, reduce the cycle time for each piece to be processed. However, the cost of the equipment is typically very much increased, also adding to the effective cost of each processed workpiece. The way out of this commercial dilemma is to look for methods and equipment for the rapid application of high pressure to large, commercial-size workpieces at a reasonable cost. Such is the subject of the present invention.
The present invention involves the rapid generation of high pressure by the thermal expansion and/or vaporization of a fluid. The basic procedures for expanding a fluid or vapor to produce high pressure has been known for at least 300 years since the time of Boyle. Likewise, rapid thermal expansion (as in an explosion) has been used many times to generate high pressures rapidly. The trick has been to achieve high pressures sufficiently rapidly to reduce cycle times; be able to maintain the hydrostatic pressures long enough to complete the process cycle; and apply such pressures to a sufficiently large volume to have commercial applications, as opposed to laboratory use; yet do all this at a reasonable cost for each workpiece processed. The development of such equipment and processes is the subject of the present invention.
We cite two typical examples from the recent patent literature for other approaches to the generation of high pressure. Estanislao (U.S. Pat. No. 4,251,488) claims to use the thermal expansion of a fluid to produce diamonds. The commercial production of diamonds is a well-established, price competitive industry using many techniques for attaining the required high pressures, of which we presume that of Estanislao is one. The reasons for the existence of several competitive technologies, we submit, is that the techniques for achieving pressures to manufacture diamonds need not be applied to large volumes. The diamond-producing process typically applies high pressure to a volume of the order of several cubic inches; quite adequate for the production of diamonds, but not for the processing of large, less-costly materials, such as steels. A way to produce high pressures economically for large volumes would have only a modest effect on the production of diamonds, but would require a novel approach to the design of the required equipment. This is one of the subjects of the present invention.
The patent of Stinger, U.S. Pat. No. 3,198,927, discloses a method for the attainment of high pressure by the rapid release of electrical energy in an expandable fluid. This is also the underlying technique used in the present invention for the rapid attainment of high pressure. As we note above, the input of heat energy to a fluid in whatever form at whatever speed has been known for many decades (centuries in some cases). However, the apparatus used by Stinger is different from that disclosed here in numerous particulars such as the use of shock waves, exploding wires and magnetic pinch effects (not needed by the present invention), and others which would be quite difficult to apply economically to the processing of large-volume workpieces. We submit, the present invention represents a practical, cost-effective approach to the reduction of cycle times for commercial-size workpieces requiring the application of high hydrostatic pressures.